In the gene's eye view, female mating frequency is difficult to understand. A substantial body of evidence, taken throughout the animal kingdom, demonstrates that females mate frequently, even when bouts of mating decrease offspring production.
This finding is counterintuitive because we would expect natural selection to remove mating behaviors which decrease fitness. However, new research suggests that frequent mating females receive fitness benefits from an unexpected source: their daughters.
Evolutionary Biologists from the University of Virginia -- Nick Priest, Laura Galloway and Deborah Roach -- manipulated the mating frequency of female fruit flies, Drosophila melanogaster, and observed how mating frequency affected the lifetime reproductive output of those females and their daughters.
They found that frequent mating decreased maternal survival and reproductive output, but increased the reproductive output of daughters.
"Frequent mating was more hazardous to females than we had suspected," explains Priest, who conducted the study as part of his doctoral research. "Increased mating frequency actually accelerated the female aging process.
But, the daughters of frequent mating mothers had enhanced offspring production." Are the benefits to daughters enough to recoup the costs of mating?
The authors evaluated their data using a model of inclusive fitness which considered the multi-generation costs and benefits of frequent mating. They found that costs to mothers were balanced by benefits to daughters such that frequency mating was neither detrimental nor beneficial.
This finding indicates that cross-generational fitness effects may play an important role in evolution of mating frequency in the fruit fly and may have a more general role in life history evolution.
That this phenomenon was discovered in the fruit fly is surprising. The fruit fly is often hailed as an example of sexual conflict, in part because males and females appeared to have conflicting optimal mating frequencies.
But, the work of Priest and colleagues shows that the genes which determine mating frequency in females might actually spread faster, despite direct costs to females, when their effects are considered over multiple generations. "Sexual conflict might still have an important role in the evolution of mating behavior," states Priest. "But, clearly, we have to consider the fitness consequences of mating over more than one generation. Males and females are not in conflict with respect to mating frequency, as they both appear to benefit from frequent mating."
The study, "Mating frequency and inclusive fitness in Drosophila melanogaster" authored by Nicholas K. Priest (University of Virginia and Indiana University), Laura F. Galloway (University of Virginia), and Deborah A. Roach (University of Virginia) was published in the January issue of the American Naturalist.
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